The greenhouse effect of CO2 has been pointed out as one of the causes of the global warming, and it has become an international urgent task to provide countermeasures to CO2 to protect the global environment against the warming. CO2 is generated by any human activities involving the combustion of fossil fuels, and there are increasing demands for suppressing emissions created thereby. Along with such an increasing demand, as countermeasures are being sought for increased productions of raw materials such as urea (a chemical application) and crude oil, and for global warming, resulting in studies energetically being made on a method for reducing and recovering CO2 contained in flue gas by bringing the flue gas emitted from a boiler in contact with an amine-based CO2 absorbent, and storing recovered CO2 without releasing it into the air. Applications include power plants that consume a large amount of fossil fuels, such as a thermal plant.
As a practical method for recovering and storing a large amount of CO2 contained in flue gas, a chemical absorption technique in which CO2 is brought into contact with a CO2 absorbent such as aqueous amine solution is a known method. In a method using the CO2 absorbent, flue gas is brought into contact with the CO2 absorbent in an absorber to reduce and to recover CO2 contained in the flue gas, the absorbent that has absorbed CO2 is heated in a regenerator so as to isolate CO2 and to regenerate the absorbent, and the absorbent is circulated back to the absorber for reuse (Patent Document 1).
FIG. 10 is a schematic of an example of a structure of a conventional CO2 recovery apparatus 1000A. As illustrated in FIG. 10, the conventional CO2 recovery apparatus 1000A includes: an absorber 1003 in which CO2-containing flue gas 1001A emitted from an industrial facility is brought into contact with CO2 absorbent 1002 to reduce CO2 contained in the CO2-containing flue gas 1001A, and a regenerator 1005 that causes the CO2 absorbent that has absorbed CO2 (hereinafter, the CO2 absorbent that has absorbed CO2 is referred to as “rich solution”) 1004 to release CO2 and regenerates the CO2 absorbent 1002. In this unit, the CO2 absorbent having CO2 reduced and being regenerated in the regenerator 1005 (hereinafter, the CO2 absorbent having CO2 reduced and being regenerated in the regenerator is referred to as “lean solution”) 1006 is reused as the CO2 absorbent 1002 in the absorber 1003.
For example, the CO2-containing flue gas 1001A that is the flue gas emitted from an industrial facility, such as a boiler or a gas turbine, is supplied to the absorber 1003 via a gas introducing line 1007. In the absorber 1003, the flue gas 1001A is brought into counter-current contact with the CO2 absorbent 1002 that is alkanolamine based, for example, in a CO2 absorbing unit 1010, and CO2 contained in the CO2-containing flue gas 1001A is absorbed by the CO2 absorbent 1002 by way of a chemical reaction (R—NH2+H2O+CO2(R—NH3HCO3). The mist of the CO2 absorbent 1002 accompanying the CO2-reduced flue gas 1001B is then collected by way of a CO2 absorbing unit demister 1011, and the CO2 absorbent 1002 is deposited in a bottom 1012 of the absorber 1003.
The CO2-reduced flue gas 1001B is then washed by being brought into gas-liquid contact with water supplied from the top of the washing unit 1013 in the washing unit 1013. At this time, water supplied from a water tank or water supplied via a washing water supply line 1014, both of which are not illustrated, is used as the water for washing, and is supplied after being cooled in a heat exchanger 1015. The mist accompanying the CO2-reduced flue gas 1001B is removed and collected at a washing unit demister 1016, and CO2-reduced flue gas 1001C having mist removed is released out of the system from a top 1017 of the absorber 1003.
The pressure of the rich solution 1004 accumulated at the bottom 1012 of the absorber 1003 is increased through a rich solution supply line 1018 via a rich solvent pump 1019, heated in a rich/lean solution heat exchanger 1020, and supplied to the regenerator 1005.
The rich solution 1004 released into the regenerator 1005 is heated by steam 1022 from a regenerating heater 1021, releasing a major portion of the CO2 and reducing CO2 concentration to a low level, and the CO2 absorbent 1002 is regenerated as the lean solution 1006. The released CO2 passes through a recovering unit 1023 and a concentrating unit 1024; and CO2 gas 1025 accompanying water vapor is released the exterior, and after the water is condensed and separated in a condenser 1026 and a separation drum 1027, CO2 gas 1028 is released out of the system. The water separated in the separation drum 1027 is supplied to the regenerator 1005 or the absorber 1003 via circulating water supply lines 1031-1 and 1031-2 as circulating water 1030.
The lean solution 1006 is cooled in the rich/lean solution heat exchanger 1020, its pressure is raised by the lean solvent pump 1032, is further cooled in the lean solvent heat exchanger 1033, and then is supplied into the absorber 1003 as the CO2 absorbent 1002.
In a reclaimer 1040, the lean solution 1006 is extracted through a lean solution supply line 1041, and retrograded substances such as salt remaining in the lean solution 1006 are extracted by way of a line 1042 and allowed to react with a basic sodium compound 1043, after which they are discharged as sludge 1044.
In FIG. 10, the CO2 recovery apparatus 1000A may be added afterward so as to recover CO2 emitted from an existing flue gas source, or may be installed together with a newly established flue gas source.
FIGS. 11 and 12 are schematics of other conventional CO2 recovery apparatuses in which a process of supplying CO2 absorbent 1002 is improved. As illustrated in FIG. 11, in another conventional CO2 recovery apparatus 1000B, semi-lean solution (hereinafter, the CO2 absorbent that has released a portion or a major portion of CO2 in the regenerator is referred to as “semi-lean solution”) 1051 is extracted at the middle portion of the regenerator 1005 via a semi-lean solution extracting line 1050. The extracted semi-lean solution 1051 is cooled in a rich/semi-lean solution heat exchanger 1052 and a heat exchanger 1053, and then is supplied into the upper side of a lower CO2 absorbing unit 1010-L so as to recover CO2. The CO2 absorbent 1002 extracted from the lower side of an upper CO2 absorbing unit 1010-U is also supplied to the upper side of the lower CO2 absorbing unit 1010-L in the absorber 1003 together with the semi-lean solution 1051 so as to recover CO2 (Patent Document 2).
Furthermore, in another conventional CO2 recovery apparatus 1000C, as illustrated in FIG. 12, the CO2 absorbent 1002-1 and 1002-2, extracted at the middle portion of the CO2 absorbing unit 1010 included in the absorber 1003 via CO2 absorbent extracting lines 1060-1 and 1060-2, respectively, is cooled in heat exchangers 1061-1 and 1061-2, respectively, and then supplied into the absorber 1003 again so as to recover CO2 (Patent Document 3).
In this manner, in operations recovering CO2 by way of the chemical absorption technique, such as those in the conventional CO2 recovery apparatuses 1000A to 1000C, the aqueous amine solution that is the CO2 absorbent 1002 is separated from CO2 by way of high-temperature steam, for example. The process of supplying the CO2 absorbent 1002 is improved, whereby the CO2 recovery efficiency is improved and, the steam (energy) consumption is decreased.    [Patent Document 1] Japanese Patent Laid-open No. H3-193116    [Patent Document 2] Specification in U.S. Pat. No. 6,800,120    [Patent Document 3] Japanese Patent No. 3416443